Richard Holdener
November 1, 2003

Like the proverbial redheaded stepchild, the 4.6 two-valve Mustang GT has always taken a back seat to the more powerful 4V SVT version. Originally, the '96 four-valve Cobra motors offered a 1.5-second, quarter-mile advantage over the weak-kneed, 215-horse GT. Things changed dramatically when SVT unleashed the supercharged monster known affectionately as the '03 Cobra.

While the new GTs can run in the high 13-second zone stock, adding a positive displacement supercharger to an already impressive four-valve-per-cylinder powerplant literally transformed the Cobra into an absolute over-achiever, capable of 12.4-second e.t.'s in bone-stock trim. The press (yours truly included) was quick to lob rave review after rave review toward the well-deserving SVT, most of which was aimed at the thunder produced by the force-fed, four-valve 281.

The two-valve 4.6 Mustang supercharger kit included everything necessary to add over 100 hp at just 6 psi. The 9 psi, intercooled kit (shown) was good for more than 150 extra horsepower.

Unfortunately, the 2V GT owners, even with the now upgraded 260-horse engine, were quickly left in the dust in terms of performance. Well, GT owners, all that is about to change--enter the new Twin Screw supercharger kit from Kenne Bell. In the spirit of "If you can't beat 'em, join 'em," Kenne Bell decided to give the 4.6 GT owners a taste of supercharged Cobra power by offering what the GT motor was missing, namely boost pressure. Oh sure, the DOHC SVT mill sports four valves per cylinder while the GT motor makes due with half that number, but the addition of the more efficient twin-screw Autorotor blower all but makes up the power difference.

If you follow MM&FF religiously, you will no doubt remember the power gains offered by the addition of the twin screw Autorotor to the equally supercharged SVT Lightning. Simply replacing the Eaton M112 roots blower with the more efficient twin-screw design upped the power to the tune of 60 horsepower. Similar gains are available from Kenne Bell for the SVT Cobra, but we now turn our attention to the GT, as our favorite redheaded stepchild is in dire need of an attitude adjustment. That adjustment comes in the form of a new supercharger kit (intercooled to boot) from Kenne Bell.

Readers may remember testing performed on the prototype of the kit some time back in MM&FF. After witnessing the power gains offered by the twin-screw compared to the traditional positive displacement roots blower, the gang at Kenne Bell recognized the potential for a dedicated kit. Taking their prototype pieces, they set out to build dedicated castings and the associated components you have come to expect of a production Kenne Bell supercharger kit.

The highlight of the new Kenne Bell was the 417 twin-screw Autorotor supercharger.

Naturally the highlight of the kit was the supercharger itself. Like all of the kits from Kenne Bell, the 4.6 GT kit is supplied boost via a twin-screw Autorotor supercharger. Testing has shown the twin-screw design to be more efficient and powerful than the more common (and less expensive) roots-style supercharger.

Due to the cost difference, roots superchargers are commonly employed on a handful of OEM applications. The SVT Cobra and Lightning are two such examples, but look for SVT (not to mention any number of other manufacturers and tuners) to jump on the twin-screw bandwagon in the very near future. After witnessing the additional power available from the design, it is no wonder so many SVT Cobra and Lightning owners are opting for the twin-screw upgrade recently made available by Kenne Bell.

Available in a number of different sizes, or more accurately displacements, Kenne Bell employed the 417 Autorotor in the 4.6 GT kit. The 417 moniker designates the displacement of the supercharger as 1.7 liters. This means each revolution of the twin-screw displaces 1.7 liters of air. To put this number into perspective, the Eaton roots supercharger used on the Lightning displaces 112 cubic inches to the 104 cubic inches of the 417. Fear not for the Autorotor, as the twin-screw design more than makes up in efficiency what it lacks in displacement.

The Kenne Bell kit was not some throw-together blower deal. Kenne Bell spent a great deal of time and money on designing dedicated manifold castings for the new kit, including this lower intake manifold.

Need more convincing? Running elevated boost levels (more on this later), the 4.6 produced over 500-wheel horsepower running the 417 Autorotor. Think about that number for a second, as that equates to more than double the factory power output of a stock GT motor. It is also a great deal more than most '03 4V Cobra motors put out. How long the stock motor is likely to last at this power level is another story, but know there is plenty of power potential in the kit for those so inclined to up the boost pressure beyond the specified limits.

As you might expect, there is more to the kit than simply bolting the supercharger onto the stock long-block. The first thing to go was the factory intake manifold. Off came the composite intake and on went a dedicated (and no doubt expensive) casting that houses an air-to-water intercooler. The 6-psi kit is sold sans intercooler, but the 9-psi kit includes the trick air-to-water system to help lower the charge temperature and eliminate any chance of detonation. Lower charge temperatures allow more aggressive tuning for maximum power.

Just like the 4V SVT Cobra and Lightning setups from the factory, the Kenne Bell 2V GT kit employed a lower intake mounted air-to-water intercooler to minimize the inlet charge temperature. Testing has shown the system to be very effective, even at elevated boost levels.

The supercharger is bolted directly to the lower intake, while another dedicated casting is used to supply air from the stock throttle body to the inlet of the supercharger. A great deal of effort went into the design of the kit to allow the system to bolt in with a minimum of fuss and to retain the majority of the factory inlet components.

Retained are the stock air filter assembly, mass air meter and throttle body along with the associated inlet tubing. Of course additional horsepower is available with the installation of a 90mm mass air meter, 75mm throttle body and dedicated Kenne Bell inlet tubing, but we'll get to that testing in a moment.

Kenne Bell 6 psi vs. Kenne Bell 9 psi
Here is where things really started to get serious. Equipped with the 9-psi intercooled Kenne Bell kit, the 4.6 thumped out an even 400 hp and a whopping 427 lb-ft of torque. The intercooler dropped the inlet charge temperature to a peak of just 130 degrees. Low-charge temperatures minimize the chance of detonation, an important fact given the higher static compression ratio of the GT motor compared to the low-compression '03 Cobra. Combine the chilling effects of the intercooler with a more efficient twin-screw supercharger and endless hours of fine tuning the air/fuel and timing curves, and you have the makings of one seriously powerful (but safe) supercharged GT motor. With gains as high as 150 hp and a like amount of torque, you'll be able to feel the effects of the intercooled boost as soon as you stop on the gas, just be aware that traction will now be in seriously short supply.

In addition to the intake manifold, supercharger and inlet casting, the supercharger kit also incorporated a compressor bypass valve and dedicated fuel system. The bypass valve is designed to allow the compressed air to re-circulate back through the supercharger under cruise (non-boosted) driving conditions. This helps greatly reduce inlet (cruise) charge temperature and minimize the pumping losses associated with driving the supercharger. Theoretically, this should help improve fuel efficiency, although the addictive qualities of boost pressure will likely more than offset the minimal gains offered by the bypass valve. The bypass valve will also eliminate any residual pressure build up caused by slamming the throttle shut at max boost and max revs.

Along with the bypass valve, the supercharger kit employed a dedicated fuel system. All that extra airflow supplied by the blower must have additional fuel, and the days of the tradition rising rate FMU are now long gone with the advent of the return-less fuel system and pulse width modulate fuel pumps. Through extensive testing, Kenne Bell was able to come up with a fuel supply system for the supercharged 4.6 that included one of their trick Boost-a-Pumps, a set of 36-pound injectors and (the real key to the system) a custom computer chip to tie it all together. The injector and fuel pump upgrades ensure sufficient fuel flow, while the new Kenne Bell chip provides the fuel (and timing) when and where it is needed on the new supercharged combination.

According to Jim Bell of Kenne Bell, the base 6-psi kit was configured to provide a solid gain of 100 hp, bringing the power output of the 4.6L 2V motor within spitting distance of a bone-stock '03 Cobra. Not one to simply take the manufacturers at their word (not that we don't trust 'em), we decided to have the gang at Kenne Bell put their money where their mouth was and provide some real data in the form of a back-to-back dyno test with their new 4.6 kit. If it really had the makings of a serpent slayer, we wanted to see the data with our own eyes.

Kenne Bell 9 psi vs. Kenne Bell 11 psi
One of the things owners love about the new '03 Cobras is the ability to dial in extra horsepower with additional boost. The new Kenne Bell kit provides GT owners with the very same ability (only easier) as the fuel system mods (boost-a-pump, injectors and custom chip) combined with the ease of pulley changes make adding power for those trips to the track a five-minute job. Need proof? Here is the result of a pulley change from the 9-psi pulley (2 7/8 inch) to a smaller 2 5/8 inch. The pulley change took only a couple of minutes, as we were able to utilize the same drive belt. Adding the 2 5/8-inch pulley upped the boost pressure by 2 psi and pushed the power peak from 400 to 428 hp, with impressive power gains registered throughout the rev range. It is this dramatic gain in average power that really helps drop e.t.'s (assuming available traction) and increase trap speeds. The increase in boost pressure from the pulley change required no changes to the programming in the chip and the peak charge temperature increased only by an insignificant 1-2 degrees (something that speaks volumes about the effectiveness of the air-to-water intercooler system employed in this kit).

To that end, we took Kenne Bell up on its offer of testing the very first production 4.6 kit on a near-stock '02 GT. The GT in question was five-speed equipped with a cat-back exhaust and no other modifications. In baseline trim, it pumped out a very impressive 247-wheel horsepower and 292 lb-ft of torque. Installing the Kenne Bell supercharger kit, including the Autorotor, injectors and Boost-a-Pump resulted in a gain of over 100 hp, as the presence of 6 psi of boost upped the power peak to 351 hp. The peak torque jumped to 366 lb-ft, with exemplary gains present from as low as 1,500 rpm. Unlike in stock form, which peaked near 5,000 rpm, the supercharged combination continued to produce more and more power right to redline. Credit the short-runner intake for the revised power curve.

Okay, so they did their homework and the 6-psi kit produced more than the advertised 100 extra horsepower. Once we had the goods on the base kit, we did what any good enthusiast would do and asked for more power, something the Kenne Bell setup was more than happy to supply. The kit was reconfigured for 9 psi, requiring the addition of the air-to-water intercooler system (that bolted in place in the lower intake manifold under the supercharger) and a change in supercharger pulley. The base kit produced 6 psi with a stock crank pulley and a 3 3/8-inch blower pulley. Stepping down in blower pulley from 3 3/8-inch to 2 7/8-inch increased the boost pressure from the base setting of 6 to 9 psi. The jump in boost upped the power output from 351-wheel horsepower to a whopping 405 hp. Note that this new power reading easily exceeded the power of a stock '03 Cobra, four valves and all. The torque peak exceeded 400 lb-ft, with a peak of 427 lb-ft at 3,500 rpm. The broad torque curve offered by the Kenne Bell kit allowed the two-valve GT motor to exceed 400 lb-ft of torque from 2,400 to 5,000 rpm. Heck, the supercharged mill pumped out 350 lb-ft at just 1,500 rpm.

Data logging supplied some interesting associated data in addition to the horsepower and torque gains offered by the new supercharger kit. Equipped in 9-psi configuration (2 7/8-inch blower pulley), the charge temperature exiting the air-to-water intercooler nudged just over 130 degrees. This was pretty impressive considering the ambient temperature was a shade over 90 degrees during the test and that the inlet air temp (in the manifold) started at 110 degrees. There is always some residual heating of the charge temperature (measured in the intake manifold) from the 180-200-degree water (coolant) temperature.

Upping the boost pressure further to a peak of 11 psi resulted in a gain in power to 428 wheel horsepower, but very little gain in charge temperature. Apparently the air-to-water intercooler was still on the job as the peak post-intercooler charge temp was still less than 135 degrees. Remember, this testing was performed with the production air-to-water system in place running ambient water (no ice water tricks).

It is also important to note that although the boost levels for the kits are listed as 6 and 9 psi, they only produce those numbers at the end of the run, spending most of their time just over 5 and 8 psi respectively, making the gains that much more impressive. Throughout the testing thus far, the chip (injectors and boost a pump) had produced a dead steady (and pump-gas safe) air/fuel ratio of 11.2-11.5:1.

Effect on Charge temp ( 9, 11, 14 psi)
Speaking of intercooler efficiency, this graph illustrates the change in post-core charge temperature at three different boost levels. The three lines represent boost pressures of 9, 11 and 14 psi along with the resulting charge temperatures. As expected, the charge temperature increases ever so slightly with the increase in boost pressure. What is most impressive is the fact that there was very little gain in charge temperature despite the sizable jump in boost pressure. Though no pre-intercooler temps were taken, the change in boost pressure from 9 to 15 psi would certainly add a minimum of 60-70 degrees (probably more), yet the intercooler had no problem shedding all that extra heat. The peak post-intercooler temperature was no higher than 140 degrees, and this was with the boost cranked up to a whopping 15 psi. You're not likely to run around on the street with 15 psi and 500 wheel horsepower, at least not for long with your stock GT motor on pump gas. The test did, however, illustrate the effectiveness of the air-to-water intercooler employed in the new Kenne Bell kit.

While testing the supercharger kit at the various boost levels, the gang at Kenne Bell noticed something interesting with respect to the stock air inlet system, namely that it was restrictive. That the stock filter assembly, mass air meter and throttle body would become restrictive on this supercharged motor is not surprising, considering the fact it was designed for use on a motor making a good 150 hp less power. Once again employing data logging, Kenne Bell was able to determine (again not surprising) that the stock inlet system became more and more restrictive as they increased the boost pressure. By installing vacuum sensors in various spots in the inlet tract, they were able to data log the vacuum present in the system feeding the supercharger. The presence of vacuum in the inlet system is an indication of a restriction, and (usually) a source of additional power if the restriction can be eliminated. Stepping up in boost from 6 to 9 psi and then finally to 11 psi resulted in a dramatic increase in vacuum. Each 2-psi change in boost resulted in a corresponding 1-inch increase in vacuum. Given the increase in vacuum present in the inlet system, they knew additional power was available even at the existing pulley (drive) ratio.

Kenne Bell proceeded to test and evaluate each individual component in the inlet system, starting with the air filter assembly and working their way to the supercharger. The stock air filter housing was replaced with a Kenne Bell cold-air inlet system, which improved the power output. Next came the installation of a 90mm mass air meter to replace the factory 80mm unit. This second step was actually a necessity, as the factory meter was pegged (5 volts) long ago. The tuning offered by the chip provided the desired air/fuel ratio and timing curve despite the elevated voltage, but the real cure was to install the larger 90mm (Ford) mass air meter and the associated change in chip to utilize the larger meter. No other parameters were changed in the chip, as the timing curve and air/fuel ratio remained consistent from run to run.

The additional flow offered by the meter was worth additional power, as were the next two modifications. Swapping out the stock throttle body for a new 75mm unit along with replacing the factory rubber inlet tube connecting the meter to the throttle body was also worth additional power. Running 6 psi, the changes were worth only 6-7 hp, but the gains increased as boost increased. Compared to the same pulley ratio run with the stock air intake system, the modifications increased the power output by 19 hp at 9 psi and a full 26 hp at 11 psi. As indicated by the vacuum present, the gains were more pronounced at the higher boost levels.

Thanks to the twin-screw Autorotor (and Kenne Bell tuning), the supercharged 2V was now pumping out 454 hp at the wheels. That readers is modified '03 Cobra power, but we weren't finished yet. With the power edging ever closer to the magical 500hp mark, we just had to go for it. Having already improved the efficiency of the intake system, we decided that more boost was the answer.

Not wanting to blow a perfectly happy supercharged 4.6L GT motor to smithereens, we decided the extra octane would be beneficial in our quest for an honest 500-wheel horsepower. With an octane-enhanced mixture, we installed a 2 3/8-inch blower pulley, thereby upping the peak boost pressure to a whopping 15 psi. Running one full bar of boost, the GT topped the 500hp mark with a peak reading of 504 hp, producing an incredible 521 lb-ft of torque along the way. Amazingly enough, we had more than doubled the power output of the (normally aspirated) baseline run of 245-wheel horsepower using the new Kenne Bell supercharger kit.

Serpent slayer indeed!